Continuous ester interchange process



April 1, 1958 J. L. VODONIK v 2,829,153

CONTINUOUS ESTER INTERCHANGE' PROCESS Filed March 4, 1957 INVENTOR JOSEPH L. VODONIK BY X /W ATTORNEY United States Patent CONTINUOUS ESTER INTERCHANGE PROCESS Joseph L. Vodonik, Rocky River, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware Application March 4, 1957, Serial No. 644,193

6 Claims. (Cl. 260-470) This invention relates to the alcoholysis of esters. More particularly, it relates to the continuous reaction of a dihydn'c alcohol with an ester of a'lower-boiling alcohol and a dicarboxylic acid, such as the reaction of ethylene glycol with dimethyl terephthalate to produce glycol esters with anessentially quantitative elimination of methyl alcohol and is a continuation-in-part of my application Serial No. 322,693, filed November 26, 1952, now abandoned.

Alcoholysis of dimethyl terephthalate with ethylene glycol can be achieved at elevated temperatures with the elimination of methyl alcohol by the batch process. How'- ever, prior to the advent of the present invention no practical commercial method was known for the continuous reaction of dimethyl terephthalate with ethylene glycol so as to eliminate methyl alcohol substantially quantitatively. This circumstance had arisen because the equilibrium conditions for the reaction:

0 o znocrnomon orno-d-Q-ii-oorn if HOCHzCH O-O-E-O omcmon enson were unfavorable under the conditions normally employed in continuous. reactors. Without removal of the products of the reaction, the equilibrium conversion for this reaction is low (about 20% at 175 C.). It was found that the equilibrium was shifted to the right by employing higher ratios of ethylene glycol to dimethyl terephthalate and that the rate of reaction was markedly increased at higher temperatures. Unfortunately, however, advantage of. both these factors could not be taken simultaneously, because the boiling point of the system fell rapidly as the glycol to dimethyl terephthalate ratio was increased up to the level of 3:1. Above this ratio the boiling point of the system remained essentially level butitwasnot much higher than ethylene glycol itself and was still below that of dimethyl terephthalate.

. It might be expected in the reaction described above where the boiling pointsof the respective materials are as follows:

Methyl alcohol; 64.5 C. Ethylene glycol 197 C. Dimethyl terephthalate 282 C. Bis-p-hydroxyethyl terephthalate 320 C. (approx).

that a reaction ata temperature slightly above the boiling point of ethylene glycol would quickly bring about the elimination of methyl alcohol by volatilization. This is true in the early stages of the reaction where concentration of the methyl alcohol in the liquid phase is quickly reduced by volatilization to a level below the equilibrium concentration of the reaction so that the controlling factor is the actual rate of reaction of the ethylene glycol with the dimethyl terephthalate. However, in the later stages of the reaction (after 50 to 75% completion), where the equilibrium concentration of methyl alcohol is rel'atively small, the rate of reaction of ethylene glycol with dimethyl terephthalate is faster than the rate at which the methyl alcohol is removed from the liquid phase through volatilization. Hence, the rate of removal of the methyl alcohol at this stage of the reaction has become the controlling factor. At still higher conversions (95% and above) the methyl alcohol content in the liquid phase is not likely to be reduced below the equilibrium concentration produced by the reaction, and, hence, the reaction iseffectively prevented from continuing to the desired completion'.

It is apparent, therefore, that when the reaction becomes materially advanced so that the reaction mixture consists primarily of bis-B-hydroxyethyl terephthalate, there is little methyl alcohol present under the then equilibrium conditions. The vapor pressure of methyl alcohol over this mixture is thus so low that further removal of it in order to obtain complete conversion to bis-[i-hydroxyethyl terephthalate' or its polymers of low degree of polymerization (DP) is impossible without taking extraordinary measures. To use an excess of ethylene glycol in the feed so as to force: the equilibrium in the desired direction suffers from the disadvantage that an equal quantity of unreacted ethylene glycol must be removed with the product from the reactor.

An object of this invention is the provision of a process for driving the equilibrium reaction ROH+R'COOR:RCOOR+R"OH to completion. Another object is to provide a continuous process wherein the reaction of a dihydric alcohol with an ester of a. relatively lower-boiling alcohol and a dicarboxylic' acid is. carried to completion. A further object isto provide a process wherein the reaction of ethylene glycol with dimethyl terephthalate to form primarily bis-,8 hydroxyethyl terephthalate or its low DP polymers may be carried out continuously on a commercial scale with essentially quantitative elimination of methyl alcohol. A still further object is to produce a bis-fi-hydroxyethyl terephthalate or a low molecular Weight polymer thereof having a degree of polymerization of less than 4 on the average in an efficient and economical manner. Other objects will become apparent in the description of the invention and in the claims.

The objects of this invention are accomplished by adjusting a reaction zone in which an alcohol and an ester are made to react to produce a second alcohol more volatile than the first and a second ester so that the ratio of the first alcohol to the sum of the acid radicals in both the first and second esters is substantially constant during the reaction and has a value of at least. 3 while continually removing the more volatile alcohol that is produced.

Surprisingly, it has been found that the reaction of ethylene glycol with dimethyl terephthalate 'to produce bis-p-hydroxyethyl terephthalate. or its low DP polymers can be carried out continuously. in a rectifying column With an essentially constant ratio of free ethylene glycol to the terephthalic acid radicals in the reflux in the column section below the feed and above the reboiler while simultaneously carrying out the steps of reaction and rectification. The ratio of. these materials in the reflux is. defined as the ratio of the mols of free ethylene glycol in the overflow from a given plate to the total mols of terephthalic acid radicals in. this overflow whether in the form of dimethyl terephthalate, bis-fl-hydroxyethyl terephthalate, its low molecular weight polymers or, the mixed methyl, hydroxyethyl ester. The ratio of these materials in the reflux may be from 3 to about 30 mols, preferably from 3 to 1'0 mols of free ethylene glycol for each mol of terephthal'icv acid radicals. By operating in thisrange, the presence of the excess free ethylene glycol favorably displaces the equilibrium of the reaction in the desired direction, acts as a carrier for the volatile alcohol produced and provides a large reservoir of available heatrfor the vaporization of the volatile alcohol. The ratio can be made less but to do so results ina less quantitative elimination of methyl alcohol or a longer re action time or both. T make the ratio higher than about involves a very highheat load on the column; preferably, for the most efiicient utilization of heat in'the column, the ratio is maintained below about 10.

Other relatively higher boiling dihydric alcohols may be used in place of ethylene glycol,and other esters of a lower-boiling alcohol and a dicarboxylic acid may be used in place of dimethyl terephthalate to produce desired glycol esters of dicarboxylic acids or their polymers of low degree of polymerization, since the process of the invention is especially adapted to the productionof such esters. The invention extends to the use of mixtures of dihydric alcohols, or mixtures of esters of dicarboxylic acids with lower-boiling alcohols, as starting materials for producing mixtures of esters of glycols and dicarboxylic acids, which in turn may be converted into valuable copolyesters.

The invention therefore comprehends a continuous process for reacting a dihydric alcohol starting material comprising at least one relatively higher-boiling dihydric alcohol with an ester starting material comprising at least one ester of a lower-boiling alcohol and a dicarboxylic acid to form an ester product comprising at least one ester of a dihydric alcohol and a dicarboxylic acid, said process comprising the substantial completion of the reaction by feeding in proportion a total of from 1.3 to 3.5 mols of said dihydric alcohol starting material and 1 mol of said ester starting material to a reaction zone containing a transesteritication catalyst, withdrawing said ester product so formedat about the same molarrate as said ester starting material is added, removing by distillation the volatile alcohol as it is formed and after equilibrium is set up maintaining at least one dihydric alcohol under reflux so that inthe reaction zone the total number of mols of free dihydric alcohol is maintained at a substantially constant ratio in the range of from 3 to 30 times the total number of mols of dicarboxylic radicals present. Preferably, the ratiois'maintained at a substantially constant value of from 3 to 10.

In order that the invention may be more readily understood, reference may be had to the drawing which sets forth in schematic form an apparatus type which may be utilized in carrying out the reaction as described. In thefollowing description the reaction between ethylene glycol and dimethyl terephthalate is used to illustrate the process of the invention.

Referring now to the figure, separate streams of the reactants coming through tubes 1 and 2 and preheated to the temperature of the liquid mixture on the feed plate 3 are fed into the upper part of the bubble-cap column 4 so as to have primarily a rectifving section above the feed plate and a reacting and rectifying section below the feed plate with the upper section of the column containing appreciably fewer plates than the lower section. The ethylene glycol and dimethly terephthalate need not be fed to the same plate and in some instances it may be desirable to feed the ethylene glycol onto a plate above the feed plate 3 for the dimethyl terephthalate. The ethylene glycol may be fed to the column cold, i. e., room temperature or above, but for purposes of economy and optimum operation in the column, it is usually desirable to have itstemperature approximate the temperature of the feed plate. Similarly, the temperature of the dimethyl terephthalate feed stream may be. any reasonable temperature above its melting point, e. g., 145 to 175 C.,

'buthere too for the reasons given above, it is usually desirable to have its temperature approximate the temperature on its feed plate. The molar ratio of ethylene Ill .4 glycol to dimethyl terephthalate as fed to the reactor may vary from 1.3 to 3.5 or even more but will preferably be about 1.7 to 2.1.

Superimposed on the column is apartial condenser 5 l to control the temperature of the vapor take-off proceeding to the total condenser 6 from which the liquor rich in methyl alcohol proceeds to suitable receivers for use in other processes or for further purification if required.

Before initiating the reaction in the column it is prefilled with ethylene glycol in an amount calculated to give the desired ratio of free ethylene glycol to terephthalic acid radicals. The hold-up in the column is so adjusted, by controlling such factors as the totalliquid volume on all the plates, the feed rate of the reactants and the mols of reflux per mol of product, that a minimum of time is required for the reactants to be processed in the column to produce at outlet 7 the desired bis-fi-hydroxyethyl terephthalate or a low molecular weight polymer of this material having a DP of 2 to 4. Usually this time is not much greater than 4 hours nor much less than onehalf hour. With particularly slow catalysts chosen because of their other favorable characteristics, the time may be somewhat longer than 4 hours, but, preferably, the time that any particular terephthalic acid radical spends in the column will on the average be less than 3 hours. A heating medium, such as p-cymene or a mixture of diphenyl and diphenyl oxide (Dowtherm) is supplied to the heating coil 8 in the reboiler 9 at such a rate as to maintain a reflux having a ratio of free ethylene glycol to the terephthalic acid radicals in the range of 3 to 30; preferably the ratio is maintained in the range 3 to 10, and in a highly preferred embodiment of the invention the ratio is maintined in the range 4 to 7. The preferred operating conditions at atmospheric pressure are further delineated by employing a temperature in the reboiler of 205 to 275 C. and preferably below 240 C. e. g., 230 C., a temperature on the plate above the reboiler of 190 C. to 215 C. and preferably 202 C., a temperature on the feed plate of C. to200 C. and preferably C. and a temperature in the overhead vapor line of 64.5 to 80 C. and preferably 70 C. Obviously ifit is desired to operate at sub-atmospheric pres sure these temperatures will fall in a lower range.

To maintain equilibrium conditions the product'tc'alculated as terephthalic acid radicals) is taken out at 7 at the same rate in mols per hour as the dimethyl tereph thalate is fed. Likewise, the ratio of ethylene glycol (combined and free) to the terephthalate in the product is maintained the same as that in the feed. When the feed ratio is 2.1 mols ethylene glycol to 1 mol of dimethyl terephthalate and the emerging product is pure bis-{3- hydroxyethyl terephthalate, there will be an excess of 0.1 mol of ethylene glycol for each mol of the terephthalate exiting. If the bis-,S-hydroxyethyl terephthalate has reacted to form a low molecular weight polymer having a DP of 2, there will be an excess of glycol amounting to 0.6 mol exiting with the product for each mol of dimethyl terephthalate fed to the column. Since the product is to be polymerized into fiber-forming or filmforming polyethylene terephthalate, it is relatively unimportant whether this product is bis-p-hydroxyethyl terephthalate or a low DP polymer of it. Whether the mixture be free ethylene glycol plus bis-fi-hydroxyethyl terephthalate or its dimer, trimer, tetramer or the like is determined by the temperature and pressure conditions at the point of product take-off as well as the molar ratio of free ethylene glycol to terephthalic acid radicals at this point.

The following example shows the marked improvement in the conversion of dimethyl terephthalate and ethylene glycol to monomer when operating with an essentially constant ratio of free ethylene glycol to terephthalic acid radicals in the reflux in the column section below the feed and above the reboiler, the desired conversion being achieved when this ratio is three or above.

go gles EXAMPLE 1 The equipment comprised a jacketed '12-p1atebubb1ecap column section 4 feet 4 inches high with a 12" inside diameter and a 3 /2 inche plate spacing. This column section was mounted on a jacketed pot type reboiler 14 inches high containing a pancake type heating coil. Mounted on the bubble-cap column section was an insulatecl packed column section having an inside diameter of 6 inches and a height of 40 inches. This space was filled with stainless steel Raschig rings V: x /2 x & The vapor from the packed section passed toa partial condenser and then proceeded to a total condenser. The vapor temperature at the exit from the partial condenser was maintained at70 C. The condensate from the total. condenser flowed to a collector. All the equipment was fabricated from stainless steel. V I

In operation the bubble-cap column'section was pro: filled with sufiicient ethylene glycol to produce the desired constant'r'atio of free ethylene glycol to terephthalic acid radicals in the reflux in the column section below the feed and above the'reboiler. Molten dimethyl tereph: thalate at a temperature of 150 C. was fedto the top plate in the bubble-cap column section at a rate of 26.7 pounds per hour. Similarly, fed to this plate at the same temperature and at a rate of 18.0 pounds per hour was the ethylene glycol containing the catalyst for the re- "action.

This catalyst, in weight percent of the dimethyl terephthalate, consisted of 0.035% zinc acetate, 0.005 lithium hydride and 0.03 antimony oxide (Sb O The jacket for the bubble-cap column section was supplied with para-cymene vapor at 190 C. The rate of monomer withdrawal from the reboiler together with excess ethyl-. ene glycol was 35.9 pounds per hour. The temperature of the monomer in the reboiler was maintained at 230 C. and the Dowtherm heating vapor inthe heating coil and in thereboiler jacketwas maintained at 265 C.

When operating in thisfashion but maintaining a ratio of free ethylene glycol to terephthalic acid radicals below 3'in the reflux in the column section below the feed and above the reboiler, a poor conversionof dimethyl terephthalate to monomer was obtained while above 3 an excellent conversion was obtained as shown in the table below:

Table Product from reboiler,

percent eon.

' Plate, below feed Second plate above Av. ratio 1 in plate reboiler column above reboiler and below feed Ratio 1 Percent con.

Percent Ratio 1 con.

1' Molar ratio of free ethylene glycol to terephthalic acid radicals. 1 Percent conversion of, dimethyl terephthalate to bis=B-hy.droxyethyl terephthalate or its low DP polymers. I u

In order to obtain the desired ratio of free ethylene glycol to terephthalic acid radicals in the'column section below the feed and above the reboilerthe column is 'prefilled with the calculated amount, Ofethylene glycol.

The feed is then introduced and operation begun. After a relatively short period this ethylene glycol is distributed throughout this section of the column and equilibrium steady state conditions persist from this time cm, The

g acetate and antimony oxide.

. I e of.35.9 pounds per hour. Once suliicient ethylene glycol produced by this means has accumulated in the column to give the desired ratio of ethylene glycol to terephthalic acid radicals, the operating conditions of the column are returned to normal and the rate of monomer take-off is maintained at the equilibrium rate of 35.9 pounds per hour as indicated in the example.

It is desirable to add a catalyst to increase the rate of reaction and it is preferred to add this catalyst to one of the feed streams either in the ethylene glycol or in the dimethylterephthalate, depending on the solubility or dispersibility characteristics of the catalyst in either material. The catalyst may be lead oxide, sodium alcoholate, potassium or beryllium metal or any of the other catalysts disclosed in U. S. 2,465,319. A particularly effective catalyst combination is lithium hydride, zinc Other highly effective catalysts are manganou's acetate and sodium acetate, which are usually used in combination with antimony oxide. By employing the apparatus and process described, a product is obtained which has essentially no unreacted dimethyl terephthalate left in it (less than 1.0% and usually less than 0.25%) and hence is particularly valu* able for polymerizing into fiberand film-forming polyethylene terephthalate by any of the known methods as disclosed in such patents as U. S. 2,465,319 and 2,534,028. Although the process has been described in connection with the drawing as having been carried out in a bubblecap column, it is obvious from the example that any of the known types of rectifying columns such as a packed column may be employed although smaller or larger columns may be required to provide the proper hold=up in the column and the proper reflux ratios in the column.-

, Combinations of packed columns and bubble-cap columns may be; employed, e. g., the use of a packed column section above the feed and a bubble-cap column section below the feedto achieve economies in space, materials of construction or other important considerations. It is likewise to be understood that a total'condenser could be used in the place of the partial condenser although this substitution would probably require the addition of an additional plate or plates in the upper section of the bubble-cap column or increasing the length of this section in a packed column. These vessels can be operated at super-atmospheric pressures or sub-atmospheric pressures but preferably for matters of economy in construction and operation it is preferred to operate at atmospheric pressure.

In the process and apparatus described the reactor can consist of only a single stage as described or it can consist of multiple stages. Where multiple stages are employed, the product exiting from the reboiler in the first stage can be fed to the top of another similar such reactor which may be operating at a different reflux ratio and diiferent temperature conditions in order to efiect the desired degree of reaction. Alternately, the multiple stages can be combined into a single column, each stage having its own reboiler and each stage main taining its own particular set of operating temperatures and reflux conditions, depending on the product to be produced. Such stages may have different column diam eters, different plate capacities, as well as other differences well known to designers of rectifying equipment s'o as to provide the desired degree of versatility. A somewhat similar eflect may be achieved by inserting partial condensers in the column at various levels so as to maintain a plurality of reaction zones, each having a sub- H stantially constant ratio of free ethylene glycol to tereacids may be used, such as dibutyl terephthalate, dimethyl isophthalate, diethyl adipate, dimethyl hexahydroterephthalate, and dimethyl sebacate. A mixture of dihydric alcohols may be used, as well as a mixture of esters of dicarboxylic acids and lower-boiling alcohols. If desired, amixture of dihydric alcohols containing a polyethylene glycol or other relatively non-volatile dihydric alcohols may be used, provided that at least one dihydric alcohol inthe mixture may be maintained under reflux. When mixed dihydric .alcohols or mixed dicarboxylic esters are used as starting materials, the ratio of free dihydric alcohol to dicarboxylic radicals in the reaction zone is calculated as the ratio of the total mols of free dihydric alcohols to the total mols of dicarboxylic radicals.

EXAMPLE 2 The equipment comprised a 19 plate bubble-cap column section 17 feet inches high with a 12% inch inside diameter and ,a 12-inch plate spacing. This column section was mounted on a jacketed pot type reboiler 27 inches high containing three sets of heating coils. Mounted on the bubble-capcolumn section was an insulated shell and tube condenser, designated a partial condenser, having a shell inside diameter of 6 inches and a height of 9 feet 11 inches. This condenser consisted of seven l-inch outside diameter tubes within the shell. The vapor from the bubble-cap column section passed through this condenser and was partially condensed after which itproceeded to a total condenser. The vapor temperature at the exit from the partial condenser was maintained at 50 The condensate fromthe total condenser flowed to a collector. All equipment was fabricated from stainless steel.

The column was brought to equilibrium over a period of several hours without prefilling it, the reactants being introduced directly into the column. Molten dimethyl terephthalate at a temperature of 150 C. was fed to the thirteenth plate in the bubble-cap column section (the plates being numbered from the bottom) at an equilibrium rate of 27.8 pounds per hour. To the sixteenth plate in the column was fed an equilibrium rate of 21.0 pounds per hour ofamixture maintained at 100 C. containing 89.9% by weight ethylene glycol, 9.9% polyethylene glycol'having an average molecular weight of 3350, 0.063% manganous acetate, and 0.090% antimony trioxide. The equilibrium rate of withdrawal of the monomer mixture was 39.6 pounds per hour.

The total pressure drop across the 19 bubble-cap plates was observed to be steady at 70 inches of water with an overhead pressure of 375 mm. of mercury absolute. The temperature profile of the column was observed to be as follows:

from ethylene glycol and polyethylene glycol, amounting to 99.8% conversion as indicated by analysis of the product from the reboiler, was obtained.

EXAMPLE 3 The column described in Example 2 was brought to equilibrium over a period of several hours without prefilling it, the reactants being introduced directly into the column. Molten dimethyl terephthalate at a temperature of 150 C. was fed to the thirteenth plate at an aquilibrium rate of 28.6 pounds per hour. To the sixteenth plate in the column was fed an equilibrium rate of 21.6 pounds per hour of a mixture maintained at. C. containing 92.2% by weight ethylene glycol, 7.3% dimethyl sodium S-sulfoisophthalate, 0.44% sodium acetate, 0.028% manganous acetate, and 0.028% antimony trioxide. The equilibrium rate of withdrawal of the monomer mixture was 40.6 pounds per hour.

The vapor temperature at the exit from the partial condenser was maintained at 50 C. The total pressure drop across the 19 bubble-cap plates was observed to be steady at 70 inches of water with an overhead pressure of 375 mm. of mercury absolute. The temperature profile of the column was observed to be as follows:

Plate No.

The temperature in the reboiler was maintained at 212 C. and the Dowtherm heating vapor in the reboiler jacket and coils was maintained at 245 C.

The ratio of the mols of free ethylene glycol to the total mols of terephthalate and sodium 5-sulfoisophthalate radicals at equilibrium, as shown by analysis of plate samples, was 9.0 at the tenth plate (96.5% conversion) and 10.5 at the fourth plate (99.4% conversion). When the column was operated in this fashion an excellent conversion of dimethyl terephthalate and dimethyl sodium S-sulfoisophthalate to a mixed monomer derived from ethylene glycol, amounting to 99.99% conversion as indicated by analysis of the product from the reboiler, was obtained.

Any departure which conforms to the principles of this invention is intended to be included within the scope of the claims below.

I claim: 1

'1. In a continuous process for reacting a dihydric alcohol starting material comprising at least one relatively higher-boiling dihydric alcohol with an ester starting material comprising at least one ester of a lower-boiling alcohol and a dicarboxylic acid to form an ester product comprising at least one ester of a dihydric alcohol and a dicarboxylic acid, the improvement which comprises substantially completing the reaction by feeding in proportion from 1.3 to 3.5, mols of said dihydric alcohol starting material and 1 mol of said ester starting material to a reaction zone containing a transesterification catalyst, withdrawing said ester product so formed at about the same molar rate as said ester starting material is added, removing by distillation the volatile alcohol as it is formed and after equilibrium is set up maintaining at least one dihydric alcohol under reflux so that in the reaction zone the total number of mols of free dihydric alcohol is maintained at a substantially constant ratio in the rangeof from 3 to 30 times the total number of mols of dicarboxylic radicals present.

2. A continuous process for substantially completing an ester interchange which comprises feeding in proportion from 1.3 to 3.5 mols of ethylene glycol and 1 mol of dimethyl terephthalate to a reaction zone containing a transesterification catalyst, withdrawing the glycol ester of terephthalic acid so formed at about the same molar rate as the dimethyl terephthalate is added to the reaction zone, removing the methyl alcohol as his formed by distillation and, after equilibrium is set up, maintaining the amount of free glycol in the reaction zone under reflux at a substantially constant molar ratio in the range of from 3 to 10 times the amount of combined tcrephthalic radicals present in both dimethyl terephthalate and ethylene glycol terephthalate.

3. In a continuous process for reacting a relatively higher-boiling dihydric alcohol with an ester of a lower boiling alcohol and a dicarboxylic acid to form an ester of the dihydric alcohol and the dicarboxylic acid, the improvement which comprises substantially completing the reaction by feeding in proportion from 1.3 to 3.5 mols of the dihydric alcohol and 1 mol of the ester of a lower boiling alcohol and dicarboxylic acid to a reaction zone containing a transesterification catalyst, withdrawing the ester of the dihydric alcohol and dicarboxylic acid so formed at about the same rate the first-mentioned ester is added, removing the volatile alcohol as it is formed by distillation and after equilibrium is set up maintaining the dihydric alcohol under reflux at a substantially constant value of from 3 to 10 times the mols of combined dicarboxylic radicals present in both esters.

4. The process of claim 1 in which the said relatively higher-boiling dihydric alcohol is ethylene glycol and the said ester starting material is dimethyl terephthalate.

5. The process of claim 1 in which the said dihydric alcohol starting material is a mixture of ethylene glycol and polyethylene glycol.

6. The process of claim 1 in which the said ester starting material contains dimethyl sodium 5-sulfoisophthalate.

No references cited. 

1. IN A CONTINUOUS PROCESS FOR REACTING A DIHYDRIC ALCOHOL STARTING MATERIAL COMPRISING AT LEAST ONE RELATIVELY HIGHER-BOILING DIHYDRIC ALCOHOL WITH AN ESTER STARTING MATERIAL COMPRISING AT LEAST ONE ESTER OF A LOWER-BOILING ALCOHOL AND A DICARBOXYLIC ACID TO FORM AN ESTER PRODUCT COMPRISING AT LEAST ONE ESTER OF A DIHYDRIC ALCOHOL AND A DICARBOXYLIC ACID, THE IMPROVEMENT WHICH COMPRISES SUBSTANTIALLY COMPLETING THE REACTION BY FEEDING IN PROPORTION FROM 1.3 TO 3.5 MOLS OF SAID DIHYDRIC ALCOHOL STARTING MATERIAL AND 1 MOL OF SAID ESTER STARTING MATERIAL TO A REACTION ZONE CONTAINING A TRANSESTERIFICATION CATALYST, WITHDRAWING SAID ESTER PRODUCT SO FORMED AT ABOUT THE SAME MOLAR RATE AS SAID ESTER STARTING MATERIAL IS ADDED, REMOVING BY DISTILLATION THE VOLATILE ALCOHOL AS IT IS FORMED AND AFTER EQUILIBRIUM IS SET UP MAINTAINING AT LEAST ONE DIHYDRIC ALCOHOL UNDER REFLUX SO THAT IN THE REACTION ZONE THE TOTAL NUBER OF MOLS OF FREE DIHYDRIC ALCOHOL IS MAINTAINED AT A SUBSTANTIALLY CONSTANT RATIO IN THE RANGE OF FROM 3 TO 30 TIMES THE TOTAL NUMBER OF MOLS OF DICARBOXYLIC RADICALS PRESENT. 